Ruroni Kenshin becomes much stronger in the second into third season with the Makoto arc, but as a long series it suffers from many of the same problems as all long anime series - namely a large amount of filler, not Bleach or Naruto bad, but still quite a bit of fluff. Secondly, the opening season had some growing pains, and the music gets better later on, but then thirdly it suffers a bit of a long, drawn-out death as the series quality declines, ending in some if I remember correctly rather pointless episodes. However when it's good, it's very good, but it will likely never make my "Must Buy" list.
Gurren Lagaan, otoh, is one I will be looking to own once I have some disposable income, which is non-existent right now for me.